1. Field of the Invention
The present invention relates in general to semiconductor memory devices and, in particular, to a semiconductor memory device having a plurality of memory cells each bit of which comprises a MOS transistor and a MOS capacitor.
2. Discussion of Background
In a dynamic random access memory (dynamic RAM) having one-bit memory cells each of which comprises a metal oxide semiconductor (MOS) transistor and a MOS capacitor, digital storage data is stored in the MOS capacitor by means of charging or discharging thereof. The amount of charge in the MOS capacitor of the memory cell largely determines the performance of the dynamic RAM. The charge Q of the MOS capacitor is given as follows: EQU Q=CV
where C is the capacitance of the MOS capacitor and V is the write voltage. Thus, when the write voltage V is decreased, the charge Q is proportionally decreased. In order to compensate for the decrease in charge, the capacitance C must be increased.
In order to increase the capacitance C, (i) the area of the conductors of the capacitor must be increased, (ii) the thickness of an insulating film as a dielectric must be reduced, and/or (iii) an insulating film having a large dielectric constant must be used. However, in order to increase the area of the conductors, the chip size of the dynamic RAM is increased in accordance with the same design rule, decreasing the product yield and increasing the manufacturing cost. On the other hand, when the thickness of the insulating film is greatly reduced, the dielectric withstand voltage of the element is decreased, resulting in degradation of element reliability. Although an Si.sub.3 N.sub.4 film, which has a dielectric constant twice that of an SiO.sub.2 film, is currently receiving a great deal of attention as an insulating film, the manufacturing process becomes complicated when this film is used and a large leakage current occurs, which degrades industrial applicability. Furthermore, no other insulating film suitable for use with the present silicon film formation techniques has yet been proposed.